To ensure the reliability of an electric power grid, the operator continually maintains a power reserve to compensate for a possible failure of electrical generation units. The power reserve is essentially an excess production capacity on standby. In normal conditions, the power generation units are run at less than 100% such that a degree of reserve power is always available. However, the maintenance of this reserve capacity is an expensive proposition since the reserve constitutes a resource that cannot be effectively monetized by the utility company.
An AC power grid will operate in normal conditions at a fixed frequency (usually 50 or 60 Hz). The frequency remains constant as long as the supplied power matches the power consumed by the load. Any sudden changes in generation or load resulting in an imbalance between generation and load will lead to a frequency instability during which the frequency deviates from its nominal value. Large frequency variations are undesirable because they could lead to equipment trip or even a system collapse.
Frequency instability events are generally caused by the sudden loss of an electrical generation unit or by the loss of a large load and are characterized by a sudden frequency variation from the frequency nominal value.
The reserve capacity in a power grid is thus tapped when the frequency drops below a certain level. Electrical generation units that supply power to the grid are equipped with a speed governor. The speed governor continuously regulates the power output of generation units to balance the generation with the load. Thus, when the frequency of the power grid varies, the speed governor responds to this variation to compensate it. For example, when the frequency is higher than normal, the speed governor will simply lower the power generated by the generation unit (therefore reducing the amount of power supplied to the grid). Alternatively, when the frequency is lower than normal, the speed governor will increase the power generation. The speed governor however has some inherent limitations. In particular, it is slow to respond since it involves certain mechanical constraints. Depending of the type of generation (hydraulic, gas, thermal, wind, etc. . . . ), some time is required for the generation unit to increase its speed up to the desired point.
System inertia is another aspect to frequency stability of the AC power grid. “Inertia” refers to the ability of the grid to buffer imbalances, such as excess power generation or power generation deficit and thus prevent significant and rapid frequency excursions. Any AC power grid has a level of inherent inertia. This inherent inertia effect is the result of the energy stored in the AC power grid that builds up or bleeds off to buffer the imbalance, depending on whether the imbalance is the result of an excess or deficit of power generation. Most of this energy is the kinetic energy of the power generators and other rotating masses. When the AC power grid experiences a significant imbalance due to a power generation deficit, the kinetic energy will be tapped and converted in electricity to feed the load, thus compensating temporarily the power generation deficit. As the kinetic energy bleeds off, the power generators and other rotating masses will slow down causing the frequency to deviate from its nominal value. The rate of deviation of the frequency is thus dependent on the rate of kinetic energy depletion.
Accordingly, from the perspective of frequency stability, some level of inertia in the power grid is desirable because it acts as a mechanism to dampen frequency variations and thus provides more time for slower frequency stabilization systems to become active.